U.S. patent application number 12/631964 was filed with the patent office on 2010-06-10 for confined crystallization multilayer films.
Invention is credited to ERIC BAER, Anne Hiltner.
Application Number | 20100143709 12/631964 |
Document ID | / |
Family ID | 42231417 |
Filed Date | 2010-06-10 |
United States Patent
Application |
20100143709 |
Kind Code |
A1 |
BAER; ERIC ; et al. |
June 10, 2010 |
CONFINED CRYSTALLIZATION MULTILAYER FILMS
Abstract
A multilayer film includes an extruded first polymer layer
confined between extruded second polymer layers. The first polymer
layer includes a high aspect ratio crystalline lamellae. The
multilayer film is substantially impermeable to gas diffusion.
Inventors: |
BAER; ERIC; (Cleveland
Heights, OH) ; Hiltner; Anne; (Cleveland,
OH) |
Correspondence
Address: |
TAROLLI, SUNDHEIM, COVELL & TUMMINO, LLP
1300 EAST NINTH STREET, SUITE 1700
CLEVELAND
OH
44114
US
|
Family ID: |
42231417 |
Appl. No.: |
12/631964 |
Filed: |
December 7, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61120140 |
Dec 5, 2008 |
|
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Current U.S.
Class: |
428/336 ;
264/112; 428/422; 428/474.4; 428/480; 428/523 |
Current CPC
Class: |
B32B 2250/05 20130101;
B29C 48/495 20190201; Y10T 428/31544 20150401; B29C 48/185
20190201; B32B 2457/20 20130101; B29C 48/154 20190201; B29K
2995/0041 20130101; B29C 48/0017 20190201; B29C 48/0018 20190201;
B29K 2995/0067 20130101; B32B 27/08 20130101; B32B 27/32 20130101;
Y10T 428/31725 20150401; B29C 48/08 20190201; B29C 48/365 20190201;
B32B 7/02 20130101; B29C 48/023 20190201; B29C 48/307 20190201;
B32B 27/30 20130101; B32B 27/36 20130101; B32B 2307/7244 20130101;
B32B 2439/70 20130101; Y10T 428/31938 20150401; B32B 2307/704
20130101; B29C 48/71 20190201; B32B 2250/24 20130101; B32B 27/365
20130101; B29C 48/387 20190201; Y10T 428/31786 20150401; B32B
2307/7242 20130101; B32B 2535/00 20130101; B29C 48/21 20190201;
B29L 2007/008 20130101; B32B 7/12 20130101; B32B 2250/42 20130101;
Y10T 428/265 20150115; B32B 27/34 20130101 |
Class at
Publication: |
428/336 ;
428/523; 428/474.4; 428/480; 428/422; 264/112 |
International
Class: |
B32B 5/00 20060101
B32B005/00; B32B 27/00 20060101 B32B027/00; B32B 27/34 20060101
B32B027/34; B32B 27/36 20060101 B32B027/36; B32B 27/30 20060101
B32B027/30; B29C 47/06 20060101 B29C047/06 |
Goverment Interests
GOVERNMENT FUNDING
[0002] This invention was made with government support under Grant
No. RES501499 awarded by The National Institute of Health. The
United States government has certain rights in the invention.
Claims
1. A multilayer film comprising a first polymer layer coextruded
with and confined between second polymer layers, the first polymer
layer comprising a high aspect ratio crystalline lamellae, the
multilayer film being substantially impermeable to gas
diffusion.
2. The multilayer film of claim 1, the first polymer layer a
substantially crystalline lamellae.
3. The multilayer film of claim 1, the lamellae extending in a
plane parallel to the first layer and the opposite second
layers.
4. The multilayer film of claim 1, the first polymer being selected
from the group consisting of polyethylenes, polypropylenes,
polyethylene oxide, polycaprolactone, polyamides, polyesters, and
polyvinylidene fluoride.
5. The multilayer film of claim 1, the first layer having an
average thickness of about 10 nm to about 500 nm.
6. The multilayer film of claim 1, the aspect ratio of the
crystalline lamellae being at least about 5.
7. The multilayer film of claim 1, comprising a plurality of
extruded first layers confined between a plurality of second
layers.
8. The multilayer film of claim 1, the second polymer layer being
immiscible or partially miscible with the first polymer layer.
9. A multilayer film comprising a plurality of extruded first
polymer layers and a plurality of extruded second polymer layers,
each first polymer layer being confined between second polymer
layers and comprising a high aspect ratio crystalline lamellae, the
multilayer film being substantially impermeable to gas
diffusion.
10. The multilayer film of claim 9, each first polymer layer
comprising a substantially crystalline lamellae.
11. The multilayer film of claim 9, the lamellae extending in a
plane parallel to the first layers and the opposite second
layers.
12. The multilayer film of claim 9, the first polymer being
selected from the group consisting of polyethylene, polyethylene
oxide, polyamide, and polypropylene.
13. The multilayer film of claim 9, the first layers having an
average thickness of about 10 nm to about 500 nm.
14. The multilayer film of claim 9, the aspect ratio of the
crystalline lamellae being at least about 5.
15. The multilayer film of claim 9, the second polymer layers being
immiscible or partially miscible with the first polymer layers.
16. A method of forming a confined crystallization multilayer film,
the method comprising: coextruding a plurality of first polymer
layers and a plurality of second polymer layer so that each first
polymer layer is sandwiched between second polymer layers, each
first polymer layer comprising a high aspect ratio crystalline
lamellae, the multilayer film being substantially impermeable to
gas diffusion.
17. The method of claim 16, each first polymer layer comprising a
substantially crystalline lamellae.
18. The method of claim 16, the lamellae extending in a plane
parallel to the first layers and the opposite second layers.
19. The method of claim 16, the first polymer being selected from
the group consisting of polyethylene, polyethylene oxide,
polyamide, and polypropylene.
20. The method of claim 16, the first layers having an average
thickness of about 10 nm to about 500 nm.
21. The method of claim 16, the aspect ratio of the crystalline
lamellae being at least about 5.
22. The method of claim 16, the second polymer layers being
immiscible or partially miscible with the first polymer layers.
Description
RELATED APPLICATION
[0001] This application claims priority from U.S. Provisional
Application No. 61/120,140, filed Dec. 5, 2008, the subject matter,
which is incorporated herein by reference.
TECHNICAL FIELD
[0003] The present invention is directed to a multilayer film and,
in particular, to a method and apparatus for forming a multilayer
film that includes a confined crystallization layer.
BACKGROUND
[0004] The rapid development of nanoscience and nanotechnology is
driving an interest in ultra-thin polymer layers with highly
controlled and selective gas barrier properties. New packaging
strategies that utilize the unique gas transport characteristics of
nanolayers could significantly address growing environmental and
energy concerns. Considering the magnitude of the need, packaging
with enhanced gas barrier and selectively could dramatically reduce
the amount of food waste, thereby reducing world hunger, greenhouse
gas generation and the load on global water and energy supplies (A.
L. Brody, Food Technology 62(6), 121 (2008)).
[0005] Crystallization is a common approach for reducing gas
permeability of polymers in the bulk (D. H. Weinkauf, D. R. Paul,
in Barrier Polymers and Structures, W. J. Koros. Ed. American
Chemical Society, Washington, D.C. 1990), pp. 60-91.).
Crystallization is a self-organization process through which
molecules are arranged in a regular order (M. D. Ward, Science 308,
1566-1567 (2005)). It is fundamental to the structural
characteristics and the physicochemical properties of many
substances, including inorganic materials, small organic molecules,
synthetic polymers and biomacromolecules (V. Cherezov, D. M.
Rosenbaum, M. A. Hanson, S. G. F. Rasmussen, F. S. Thian. T. S.
Kobilka, H. J. Choi, P. Kuhn, W. J. Weis. B. K. Kobilka. R. C.
Stevens, Science 318, 1258-1265 (2007); S. G. F. Rasmussen, H. J.
Choi, D. M. Rosenbaum. T. S. Kobilka. F. S. Thian, P. C. Edwards,
M. Burghammer, V. R. P. Ratnala, R. Sanishvili, R. F. Fischetti, G.
F. X. Schertler, W. J. Weis, B. K. Kobilka. Nature 450, 383-U384
(2007)).
[0006] The two-dimensional crystallization of polymers is
conventionally studied with polymeric thin films or block
copolymers that contain at least one crystallizable block. In the
former, crystallizable layers with nanometer to submicron
thicknesses are prepared by a solution process such as
Langmuir-Blodgett, spin-coating techniques. These approaches are
limited by the solvent requirement and by the small amount of
material that can be fabricated. In the latter, a lamellar phase
morphology on the length scale of a few tens of nanometers can be
achieved as a consequence of microphase separation of the
dissimilar blocks below the order-disorder transition temperature
(TODT) (F. S. Bates, G. H. Fredrickson, Annual Review of Physical
Chemistry 41,525-557 (1990)). Shear-alignment is often necessary to
construct well-defined layering with a uniformly oriented,
micron-scale phase morphology (Z.-R. Chen, J. A. Kornfield, S. D.
Smith, J. T. Grothaus, M. M. Satkowski Science 277,1248-1253
(1997)).
SUMMARY OF THE INVENTION
[0007] The present invention relates to a multilayer film that
includes a first polymer layer coextruded with and confined between
second polymer layers. The first polymer layer includes a high
aspect ratio crystalline lamellae. The multilayer film can be
substantially impermeable to gas diffusion.
[0008] In an aspect of the invention, the first polymer layer can
form a substantially crystalline lamellae. The lamellae can extend
in a plane parallel to the first layer and the second layers. The
first polymer layer can include a first polymer that can readily
form a substantially crystalline lamellae upon confinement between
the second polymer layers. Examples of polymers that can be used as
the first polymer are polyethylenes, polypropylenes, polyethylene
oxide, polycaprolactone, polyamides, polyesters, and polyvinylidene
fluoride.
[0009] The second polymer layer can include a thermoplastic or
thermoformable second polymer that is immiscible or partially
miscible with the first polymer and can readily confine the first
polymer layer upon coextrusion. Examples of polymers that can be
used as the second polymer are polystyrene, polycarbonate,
polymethylmethacrylate, low-density polyethylene, polyamides,
ethylene-co-acrylic acid, and polyoxymethylene.
[0010] The first polymer layer can have a thickness that is
effective to promote crystallization of the first polymer layer.
For example, the first polymer layer can have an average thickness
of about 10 nm to about 500 nm. The aspect ratio of the
substantially crystalline lamellae can be at least about 5, for
example, at least about 10 to about 1000.
[0011] In another aspect of the invention, the multilayer film can
include a plurality of alternating coextruded first polymer layers
and coextruded second polymer layers. Each first polymer layer can
be confined between second polymer layers. The alternating first
layers and second layers can be arranged in a stack configuration
and formed using a layer-multiplying forced coextrusion
process.
[0012] The present invention also relates to a method of forming a
confined crystallization multilayer film. The method includes
coextruding a plurality of first polymer layers and a plurality of
second polymer layer so that each first polymer layer is sandwiched
between second polymer layers. Each first polymer layer comprises a
high aspect ratio substantially crystalline lamellae. The
multilayer film can be substantially impermeable to gas
diffusion.
[0013] In an aspect of the invention, the multilayer film can
comprise a stacked series of substantially crystalline lamellae.
The lamellae can extend in a plane parallel to the first layers and
the second layers. The first polymer layers can include a first
polymer that can readily form a substantially crystalline lamellae
upon confinement between the second polymer layers. Examples of
polymers that can be used as the first polymer are polyethylenes,
polypropylenes, polyethylene oxide, polycaprolactone, polyamides,
polyesters, and polyvinylidene fluoride.
[0014] The second polymer layers can include a thermoplastic or
thermoformable second polymer that is immiscible or partially
miscible with the first polymer and can readily confine the first
polymer layers upon coextrusion. Examples of polymers that can be
used as the second polymer are polystyrene, polycarbonate,
polymethylmethacrylate, low-density polyethylene, polyamides,
ethylene-co-acrylic acid, and polyoxymethylene.
[0015] The first polymer layers can each have a thickness that is
effective to promote crystallization of the first polymer layers.
For example, each first polymer layer can have an average thickness
of about 10 nm to about 500 nm. The aspect ratio of the
substantially crystalline lamellae can be at least about 5, for
example, at least about 10 to about 1000.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 illustrates a schematic view of a multilayer film in
accordance with an aspect of the invention.
[0017] FIG. 2 illustrates an image of a polyethylene oxide (PEO)
single crystal formed by continuous melt processing of a
polyethylene oxide (PEO)/ethylene-co-acrylic acid (EAA) multilayer
film.
[0018] FIG. 3 illustrates illustrates the 2D WAXS patterns and 2D
SAXS patterns of polycaprolactone (PCL)/polystyrene (PS) multilayer
films with different polycaprolactone layer thicknesses.
[0019] FIG. 4 illustrates plots showing the effect of confined
layer thickness on crystal orientation function for polyethylene
oxide (PEO)/ethylene-co-acrylic acid (EAA) multilayer films and
polycaprolactone (PCL)/polystyrene (PS) multilayer films.
[0020] FIG. 5 illustrates plots showing the effect of confined
layer thickness on gas permeability for polyethylene oxide
(PEO)/ethylene-co-acrylic (EAA) acid multilayer films, polyethylene
oxide (PEO)/polystyrene (PS) multilayer films, and polycaprolactone
(PCL)/polystyrene (PS) multilayer films.
[0021] FIG. 6 is a schematic illustration of a layer-multiplying
coextrusion process for forced-assembly of polymer nanolayers in
accordance with an aspect of the invention.
[0022] FIG. 7 is a schematic illustration of a layer-multiplying
coextrusion for forced-assembly of polymer nanolayers in accordance
with another aspect of the invention.
[0023] FIG. 8 illustrates plots showing the effect of layer
thickness on oxygen permeability. The PEO and EAA layers had the
same thickness, and the layer thickness was varied by changing the
number of coextruded layers and/or the film thickness while
maintaining the composition at 50/50 v/v. The dashed line indicates
the calculated P// from eq(1).
[0024] FIG. 9 illustrates plots showing the effect of layer
thickness on oxygen permeability. Films that varied in both the
composition ratio and the layer thickness were tested and the
oxygen permeability of the PEO layer P.sub.PEO.eff was calculated
from eq. (2). The dashed line indicates PPEO. The open symbol is
for a film with PEO layer breakup. The solid lines are drawn to
guide the eyes.
[0025] FIG. 10 illustrates AFM phase images of partial cross
sections of the coextruded EAA/PEO films. (A) A low resolution
image of an EEA/PEO film with 50/50 composition, 33 alternating
layers and nominal PEO layer thickness of 3.6 .mu.m shows the
continuity of the coextruded layers. (B) A higher resolution image
reveals the spherulitic morphology of the 3.6 .mu.m-thick PEO
layer. (C) A low resolution image of an EAA/PEO film with 70/30
composition, 1025 alternating layers and nominal PEO layer
thickness of 110 nm shows the continuity of the coextruded layers.
(D) A higher resolution image of the 100 nm-thick PEO layers shows
the effect of confinement with crystallization of the PEO layers as
stacks of 3-5 long thin lamellae oriented in the plane of the
layer. (E) A high resolution image of an EAA/PEO film with 90/10
composition, 1025 alternating layers and nominal layer thickness of
20 nm shows that most of the PEO layers crystallized as single,
extremely long lamellae. The EAA layers and PEO layers are
identified by arrows.
[0026] FIG. 11 illustrates pole figures of normals to the (120) and
(032) planes of the PEO monoclinic crystals. The X-ray beam is
along the film extrusion direction and the pole is along the film
normal direction. (A) There is no preferred orientation of the PEO
crystals in the control film except for some very faint orientation
due to the extrusion. (B) Orientation of PEO crystals in 3.6 .mu.m
PEO layers is also very weak. (C) Orientation of the (120) planes
perpendicular to the layer plane and orientation of the (032)
planes at 67.degree. confirm that the lamellar PEO crystals are
oriented with the fold surfaces parallel to the layer plane in a
film with 50/50 composition, 1025 alternating layers and nominal
PEO layer thickness of 110 nm. (D) Orientation of the lamellae in
20 nm PEO layers is even stronger as judged from the narrower rings
in the (120) and (032) pole figures of films with 90/10 EAA/PEO
composition and 1025 alternating layers.
[0027] FIG. 12 illustrates 2D SAXS patterns of EAA/PEO films with
different PEO layer thicknesses. The patterns are measured along
the normal direction (ND) and the extrusion direction (ED): (A) 3.6
.mu.m PEO layers; (B) 110 nm PEO layers; and (C) 20 nm PEO layers.
The PEO first-order, PEO second-order and EAA first-order peaks are
marked with arrows. The scale bar in (A) defines the scattering
vector q scale. The intense meridianal streak in the ED patterns is
due to grazing incidence scattering.
[0028] FIG. 13 illustrates the 2D WAXS patterns of EAA/PEO films
with different PEO layer thicknesses. (A) The ND and ED patterns
from the 3.6 .mu.m PEO layers confirm the usual monoclinic crystal
form of PEO and show the isotropic orientation of the unit cell.
(B) The ND and ED patterns from the 110 nm PEO layers show a high
degree of orientation with the c-axis vertical to the layer plane.
(C) Arcs in the WAXS pattern of 110 nm PEO layers are sharpened to
spots in the WAXS pattern of 20 nm PEO layers.
DETAILED DESCRIPTION
[0029] The present invention relates to a multilayer film that
includes at least one confined crystallization layer. The confined
crystallization layer(s) can be formed by forced coextrusion of a
first crystallizable polymer material and a second polymer
material. The coextruded first crystallizable polymer material can
form a plurality of first crystallization polymer layers that are
confined or sandwiched between second polymer layers. Unexpectedly,
it was found that the morphology of the first polymer layers
changes as they are made progressively thinner using a
layer-multiplying process. At thicknesses on a nano-scale level
(e.g., about 5 nm to about 500 nm), each first polymer layer, as a
result of the reduced thickness and forced confinement, can
crystallize as a high aspect ratio lamellae that resembles single
large, impermeable crystals.
[0030] The resulting multilayer film with confined crystallization
layer can be substantially impermeable to gas diffusion and provide
barrier properties that allow the multilayer film to be used in
applications where selective diffusion of gases is desired. Such
applications can include, for example, food packaging applications
where it is desirable to prevent oxygen from diffusing into the
package and carbon dioxide from diffusing out of the package,
bladders for tubes or bellows, medical applications, as well as
electronic display applications where it is desirable to prevent
diffusion of gas.
[0031] FIG. 1 is a schematic illustration of a multilayer film 10
in accordance with an aspect of the invention. The multilayer film
10 in this aspect comprises alternating first crystallization
polymer layers 12 and second polymer layers 14. The second polymer
layers 14 confine or sandwich each first crystallization polymer
layers 12. Each first polymer layer can include a high aspect ratio
lamellae that is continuous and extends substantially parallel to
the first layers and the second layers. By "high aspect ratio", it
is meant an aspect ratio of at least about 5, for example, at least
about 100. In some aspects of the invention, the aspect ratio of
the substantially crystalline lamellae can be about 10 to about
1000. In other aspects of the invention, the aspect ratio of the
substantially crystalline lamellae can be about 100 to about
150.
[0032] The first polymer material used to form the confined first
crystallization layers can include any thermoplastic or
thermoformable polymer material that can be readily coextruded and
form crystals upon confinement between the second polymer layers.
Examples of polymers that can be used as the first polymer are
polyethylenes, polypropylenes, polyethylene oxide,
polycaprolactone, polyamides, polyesters, and polyvinylidene
fluoride.
[0033] By way of example, the polymeric material used to form the
confined first crystallization polymer layers can be polyethylene
oxide. As illustrated in FIG. 2, it was found that layers of
polyethylene oxide having an average thickness of about 10 nm to
about 20 nm that are confined between ethylene-co-acrylic acid
(EAA) can form single, high aspect ratio (e.g., an aspect ratio of
at least about 120) lamellae that resemble single large,
impermeable crystals.
[0034] The second polymer material that is used to form the second
polymer layers, which confine the first polymer layers, can include
any thermoplastic or thermoformable polymer material that can be
readily coextruded with the first polymer material. In an aspect of
the invention, the second polymer can include those thermoplastic
or thermoformable polymers that are immiscible or partially
miscible with the first polymer upon coextrusion. In another aspect
of the invention, the second polymer can comprise a polymer that
solidifies at a higher temperature than the first polymer to
provide confinement of the first polymer layers. Examples of
polymer material that can be used as the second polymer material
include polyethylene naphthalate and isomers thereof, such as 2,6-,
1,4-, 1,5-, 2,7-, and 2,3-polyethylene naphthalate; polyalkylene
terephthalates, such as polyethylene terephthalate, polybutylene
terephthalate, and poly-1,4-cyclohexanedimethylene terephthalate;
polyimides such as polyacrylic imides; polyetherimides; styrenic
polymers, such as atactic, isotactic and syndiotactic polystyrene,
.alpha.-methyl-polystyrene, para-methyl-polystyrene;
polycarbonates, such as bisphenol-A-polycarbonate (PC);
poycaprolactone; poly(meth)acrylates such as poly(isobutyl
methacrylate), poly(propyl methacrylate), poly(ethyl methacrylate),
poly(methyl methacrylate), poly(butyl acrylate) and poly(methyl
acrylate) (the term "(meth)acrylate" is used herein to denote
acrylate or methacrylate); ethylene/acrylic acid copolymers;
cellulose derivatives such as ethyl cellulose, cellulose acetate,
cellulose propionate, cellulose acetate butyrate, and cellulose
nitrate; polyalkylene polymers, such as polyethylene,
polypropylene, polybutylene, polyisobutylene, and
poly(4-methyl)pentene; fluorinated polymers such as perfluoroalkoxy
resins, polytetrafluoroethylene, fluorinated ethylene-propylene
copolymers, polyvinylidene fluoride, and
polychlorotrifluoroethylene; chlorinated polymers such as
polydichlorostyrene, polyvinylidene chloride and polyvinylchloride;
polysulfones; polyethersulfones; polyacrylonitrile; polyamides;
polyvinylacetate; polyetheramides. Copolymers can also be used and
include, for example, styrene-acrylonitrile copolymer (SAN),
containing between 10 and 50 wt %, preferably between 20 and 40 wt
%, acrylonitrile, styrene-ethylene copolymer; and
poly(ethylene-1,4-cyclohexylenedimethylene terephthalate) (PETG).
In addition, the second layers can include blends of two or more of
the above-described polymers or copolymers. In an aspect of the
invention, the second polymer can be selected from the group
consisting of polystyrene, polycarbonate, polymethylmethacrylate,
low-density polyethylene, polyamides, ethylene-co-acrylic acid, and
polyoxymethylene
[0035] The multilayer film can be fabricated using these materials
in a multilayer forced coextrusion method. The method can yield a
flexible large film or sheet of multilayer structure. The thickness
of the individual first confined crystallization layers can be such
that each first layer forms a substantially crystalline lamellae.
By substantially crystalline lamellae, it is meant that each first
polymer layer is at least about 60% crystalline, at least about 70%
crystalline, at least about 80% crystalline, at least about 90%
crystalline, at least about 95% crystalline, or at least about 99%
crystalline. This thickness can be on a nano-scale level and be,
for example, from about 5 nanometers to about 1000 nanometers, from
about 10 nanometers to about 500 nanometers, or from about 10
nanometers to about 20 nanometers. The thickness of each first
layer will depend on the individual polymer material used form the
first layers and can be readily selected to optimize
crystallization properties (i.e., formation of high aspect ratio
lamellae). In an aspect of the invention, the thicknesses of the
first polymer layers should be such that a high aspect ratio
crystalline lamellae is formed for each first polymer layer but not
so thin that the first polymer layers readily break-up or fracture
upon coextrusion or after confinement.
[0036] By way of example, FIG. 3 shows the crystal orientation as a
function of thickness of a confined polycaprolactone layer of a
multilayer film comprising polycaprolactone layers coextruded with
and confined between polystyrene layers. The images show that as
the thickness of the polycaprolactone layers decrease to about 75
nm the polycaprolactone layers transition from spherulitic to
flat-on "single crystal" lamellae. FIG. 4 illustrates plots showing
the crystal orientation as a function of confined layer thickness
of a polycaprolactone/polystyrene coextruded multilayer film and a
polyethylene oxide/ethylene-co-acrylic acid coextruded multilayer
film. The plots show that the orientation function of the confined
polycaprolactone layers and polyethylene oxide layers reduced
almost linearly with decreasing layer thickness from isotropic to
flat on lamellae.
[0037] Crystals are generally considered to be impermeable to small
gas molecules, and gas transport is seen as occurring through the
amorphous regions of the polymer. FIG. 5 shows that as the confined
layer thickness of, respectively, a
polyethyleneoxide/ethylene-co-acrylic acid coextruded multilayer
film, a polyethyleneoxide/polystyrene coextruded multilayer film,
and a polycaprolactone/polystyrene coextruded multilayer film is
reduced, crystallinity of the confined layers increase, and
permeability of the confined layer decreases. Reduction in
thickness and crystallization in a confined space resulted in an
unusual crystalline morphology that endowed the confined layers
with exquisitely low gas permeability.
[0038] The thickness of the individual second layers used to
confine the first layers can be on a nano-scale level. The
thicknesses of these layers can be, for example, from about 5
nanometers to about 1000 nanometers, from about 10 nanometers to
about 100 nanometers, or from about 10 nanometers to about 20
nanometers.
[0039] In one aspect of the invention, the multilayer film can made
of two alternating layers (ABABA . . . ) of the first polymer
material referred to as component "(a)" and a second polymer
material referred to as component "(b)". The components (a) and
(b), may be the same or different and form a multilayer structure
represented by formula (AB)x, where x=(2)n, and n is the number of
multiplier elements. At least one of components (a) and (b)
comprises a crystallizable polymer. It should be understood that
the multilayer structure of the invention may include additional
types of layers. For example, these other layers can include tie
layers, adhesive layers, and/or other polymer layers. The
components of the various alternating layers may be the same or
different as long as at least one component includes a
crystallizable polymer. For instance, a three component structure
of alternating layers (ABCABCA . . . ) of components (a), (b) and
(c) is represented by (ABC)x, where x is as defined above.
[0040] The multilayer polymer film layer can be prepared by
microlayer coextrusion of the two polymer materials. Nanolayers are
comprised of alternating layers of two or more components with
individual layer thickness ranging from the microscale to the
nanoscale. A typical multilayer coextrusion apparatus is
illustrated in FIGS. 6 and 7. The two component (AB) coextrusion
system consists of two 3/4 inch single screw extruders each
connected by a melt pump to a coextrusion feedblock. The feedblock
for this two component system combines polymeric material (a) and
polymeric material (b) in an (AB) layer configuration. The melt
pumps control the two melt streams that are combined in the
feedblock as two parallel layers. By adjusting the melt pump speed,
the relative layer thickness, that is, the ratio of A to B can be
varied. From the feedblock, the melt goes through a series of
multiplying elements. A multiplying element first slices the AB
structure vertically, and subsequently spreads the melt
horizontally. The flowing streams recombine, doubling the number of
layers. An assembly of n multiplier elements produces an extrudate
with the layer sequence (AB)x where x is equal to (2)n and n is the
number of multiplying elements. It is understood by those skilled
in the art that the number of extruders used to fabricate the
structure of the invention equals the number of components. Thus, a
three-component multilayer (ABC . . . ), requires three
extruders.
[0041] The multilayer film of the present invention preferably have
at least 3 layers, for example, at least about 30 layers, 50
layers, 100 layers, or 1000 layers, including any number of layers
within that range. In one example, the multilayer film of the
present invention has from 50 to 10000 layers. In another example,
the multilayer structure is in the form of film or sheet. By
altering the relative flow rates or the number of layers, while
keeping the film or sheet thickness constant, the individual layer
thickness can be controlled. The multilayer film or sheet has an
overall thickness ranging from 10 nanometers to 1000 mils,
preferably from 0.1 mils to 125 mils and any increments therein.
Further, the multilayer films may be formed into a number of
articles by, for example, thermoforming, vacuum forming, or
pressure forming. Further, through the use of forming dies, the
multilayer films may be formed into a variety of useful shapes
including profiles, tubes and the like.
[0042] The following examples are for the purpose of illustration
only and are not intended to limit the scope of the claims, which
are appended hereto.
Example
[0043] In the present example, crystalline polyethylene oxide (PEO)
with Mw=200 kg/mol was coextruded with poly(ethylene-co-acrylic
acid) (EAA), a copolymer with 9.7 wt % acrylic acid and with much
lower crystallinity. Films of 50 .mu.m, 130 .mu.m or 260 .mu.m in
thickness with 33, 257 or 1025 alternating EAA and PEO layers were
coextruded (C. D. Mueller, S. Nazarenko, T. Ebeling, T. L. Schuman,
A. Hiltner, E. Baer, Polym, Eng, Sci, 37, 355-362 (1997); T. E.
Bernal-Lara, A. Ranade, A. Hiltner, E. Baer, in Mechanical
Properties of Polymers Based on Nanostructure, 1st edition, G. H.
Micheler, F. Balta-Callaja, Eds. (CRC press, Boca Raton, Fla.
2005), pp. 629-682). Control films of PEO and EAA were also
extruded. The composition (vol/vol) was varied as EAA/PEO 50/50,
70/30, 80/20 and 90/10. The resulting nominal PEO layer thickness,
which was calculated from the number of layers, the composition
ratio, and the film thickness, varied from 3.6 .mu.m to 8 nm. The
films were stored in a desiccator to prevent moisture absorption.
The size-scale effect on crystalline morphology and gas
permeability were observed as the PEO layers were made thinner and
the confinement by the EAA layers approached the nanoscale.
Layer-Multiplying Coextrusion
[0044] Films with alternating poly(ethylene oxide) (PEO) and
poly(ethylene-co-acrylic acid) (EAA) layers with EAA outer layers
were fabricated using the layer multiplication process described
previously. The schematic drawing of layer-multiplying coextrusion
in FIGS. 6 and 7 show how a series of n multiplying elements
combines two dissimilar polymers as 2(n+1) alternating layers. With
an ABA type of feedblock, an assembly of n die elements produces
2(n+1)+1 layers with polymer A layers on both outer sides of the
film. The extruder, multipliers and die temperatures were set to
190.degree. C. to ensure matching viscosities of the two polymer
melts. Multilayered films with 33, 257 and 1025 alternating EAA and
PEO layers were coextruded as films of various thicknesses and
various composition ratios (vol/vol) including (EAA/PEO) 50/50,
70/30, 80/20 and 90/10. The nominal layer thickness was calculated
from the number of layers, the composition ratio and the film
thickness (Table 1). The films were stored at ambient temperature
in desiccators to prevent moisture absorption.
TABLE-US-00001 TABLE 1 Film composition, number of layers, film
thickness and nominal PEO layer thickness of EAA/PEO films EAA/PEO
Number of Nominal PEO layer (v/v) Layers Film Thickness (.mu.m)
Thickness (nm) 0/100 1025 110 -- 50/50 33 115 3600 70/30 33 110
2060 50/50 257 282 1100 50/50 257 130 510 70/30 257 120 280 50/50
257 46 180 50/50 1025 127 125 70/30 1025 260 110 70/30 257 38 90
70/30 1025 119 70 80/20 1025 133 50 50/50 1025 47 45 70/30 1025 51
30 80/20 1025 61 25 90/10 1025 107 20 90/10 1025 42 8 100/0 1025
121 --
Materials and Methods
[0045] Poly(ethylene oxide) (PEO) with molecular weight of 200
kg/mol (PolyOx WSR N-80) and ethylene acrylic acid copolymer (EAA)
with 9.7 wt % acrylic acid (Primacor1410) were obtained from The
Dow Chemical Company. Both EAA and PEO were dried under vacuum
before processing.
[0046] Oxygen permeabilities at 23.degree. C., 0% relative humidity
and 1 atm pressure were measured with a MOCON OX-TRAN 2/20. The
instrument was calibrated with National Institute of Standards and
Technology certified Mylar film of known O2 transport
characteristics. The specimens were carefully conditioned in the
instrument, as described previously (D. J. Sekelik, E. V. Stepanov,
S. Nazarenko, D. Schiraldi, A. Hiltner, E. Baer, Polym. Sci. Pt.
B-Polym. Phys. 37, 847-857 (1999)). The O2 permeability P was
calculated from the steady state flux.1 as
P = J l .DELTA. p ##EQU00001##
[0047] where l is the film thickness and .DELTA.p is the difference
of the oxygen partial pressure between upstream and downstream.
[0048] Differential scanning calorimentry (DSC) was conducted with
a Perkin-Elmer DSC-7 at a heating rate 10.degree. C. min-1. The
crystallinity calculated from .DELTA.Hm was 78 wt % for PEO and 34
wt % for EAA using the heat of fusion)(.DELTA.H.degree. values of
197 J g-1 for PEO crystals (C. Campbell, K. Viras, M. J.
Richardson, A. J. Masters. C. Booth, Makromol. Chem. 194, 799-816
(1993)) and 290 J g--1 for polyethylene crystals (B. Wunderlich,
Macromolecular Physics (Academic Press: New York, 1980), vol.
3,42).
[0049] Embedded films were microtomed through the thickness at
-75.degree. C. with a cryo-ultramicrotome (MT6000-XL from RMC) and
cross-sections were examined with an atomic force microscope (AFM)
in order to visualize the layers and the morphology inside layers.
Phase and height images or the cross-section were recorded
simultaneously at ambient temperature in air using the tapping mode
of the Nanoseope IIIa MultiMode scanning probe (Digital
Instruments).
[0050] Small-angle X-ray scattering (SAXS) measurements were
carried out using an inhouse set-up with rotating anode X-ray
generator (Rigaku RU 300, 12 kW) equipped with two laterally graded
multilayer optics in a side-by-side arrangement, giving a highly
focused parallel beam of monochromatic Cu K.alpha. radiation
(.lamda.=0.154 nm). The monochromatic X-ray beam operated at 50 kV
and 100 rnA was collimated using three pinholes and the diameter of
X-ray beam at sample position was approximately 700 .mu.m. For the
collection of ED and TO SAXS patterns, since the dimensions of the
films (42.about.282 .mu.m thick and 2 mm wide) were smaller than
the collimated X-ray beam, X-ray beam was irradiated along ED and
TD at an angle of .about.3.degree. to avoid total reflection. The
critical angle for the total reflection is usually the order of a
few tenths of a degree. On the other hand, in order to collect ND
SAXS patterns, X-ray beam was irradiated along the direction
parallel to ND of multilayered films. Two dimensional (2D) SAXS
were collected by using a 2D gas filled multiwire detector (Rigaku)
with a spatial resolution 1024.times.1024 pixels. The X-ray
exposure times for ED, TD and ND SAXS patterns were all 9 hours. A
sample-to-detector distance was 1.5 m and the scattering vector q
was calibrated using Silver Behenate (AgBe) standard, which had
(001) peak position at q=1.076 nm-1. A beamstop placed in front of
the area detector allowed monitoring the intensity of the direct
beam. Based on the intensity of direct beam, all SAXS images were
corrected for background scattering, dark current and sample
absorption.
[0051] Wide angle X-ray scattering (WAXS) measurements were
performed using a Statton camera coupled to a Philips PW 1830 X-ray
generator (Cu K.alpha. radiation, .lamda.=0.154 nm) operated at 30
kV and 35 mA. The collimated beam diameter was 250 .mu.m. 2D WAXS
images were collected using imaging plate and exposed imaging
plates were read with a Fujifilm FDL5000 image plate reader. The
sample-to-detector distance was 60 mm and the diffraction angle was
calibrated using CaF2, standard. Several film pieces were stacked
and glued with isocyanate 10 s glue. The thickness of the stack was
approximately 0.5 mm. The stacks were exposed in three orthogonal
directions. For directions in the plane of the film, the stack was
sectioned perpendicular to the plane of the film to obtain the
dimension of 1 mm in the X-ray beam direction.
[0052] The orientation of crystalline phase of PEO in the
multilayered films was further studied by means of X-ray
diffraction with pole figures. For overview of this technique, see
Ref. (L. E. Alexander, X-Ray Difraction Methods in Polymer Science
(Wiley: New York, 1969)). A WAXS system consisting of a
computer-controlled pole figure device associated with a wide-angle
goniometer coupled to a sealed tube X-ray generator operating at 50
kV and 30 mA (Philips) was used in this study. The X-ray beam
consisted of Cu K.alpha. radiation filtered electronically and by
Ni filter. The specimens in the form of sandwiched films approx.
0.5 mm thick were assembled with extrusion direction vertical. The
(120) and (032) crystal planes of commonly found monoclinic form of
PEO were analyzed (diffraction maxima centered around
2.theta.=19.2.degree. and 23.3.degree., respectively) and the
respective pole figures were constructed. Experimental diffraction
data were corrected for background scattering, sample absorption
and defocusing of the beam. All pole figures were plotted with the
POD program (Los Alamos National Lab, NM), Other details of the
experimental procedure were described elsewhere (M. Pluta, Z.
Bartczak, A. Galeski, Polymer 41,2271-2288 (2000)).
Results
[0053] Permeability to small molecules is an important performance
property of polymer films. However, a deeper analysis of the gas
transport characteristics provides a probe into the solid state
structure (A. Hiltner, R. Y. F. Liu, Y. S. Hu, E. Bacr, J. Polym.
Sci. Pt. B-Polym. Phys. 43, 1047.1063 (2005)), especially if used
in combination with other solid state characterization techniques.
Using commercial instruments from Macon (D. J. Sekelik, E. V.
Stepanov, S. Nazarenko, D. Schiraldi, A. Hiltner, E. Baer, J.
Polym. Sci. Pt. B-Polym. Phys. 37,847-857 (1999)), the oxygen
permeability (P) was first measured on films with the EAA/PEO 50/50
composition. In this set of experiments, the PEO and EAA layers had
the same thickness, and the layer thickness was varied by changing
the number of coextruded layers and the film thickness while
maintaining the composition at 50/50. The results are plotted in
FIGS. 8 as a function of layer thickness. Surprisingly, the oxygen
permeability decreased steadily as the EAA and PEO layers became
thinner. The permeability of a film with 45 nm-thick layers was
about 1.5 orders of magnitude lower than the permeability of a film
with 3.6 .mu.m-thick layers. A comparable reduction was found with
carbon dioxide permeability.
[0054] The series model for layered assemblies gives the gas
permeability as
P // = ( .theta. PEO P PEO + 1 - .theta. PEO P EAA ) - 1
##EQU00002##
[0055] where .phi..sub.PEO is the volume fraction of PEO, and
P.sub.PEO and P.sub.EAA are the permeabilities of PEO and EAA
extruded control films, respectively. Using the determined values
of 0.38 barrer and 2.30 barrer for P.sub.PEO and P.sub.EAA,
equation (1) gave the permeability of an EAA/PEO 50/50 layered
assembly as 0.65 barrer. However, only the film with the thickest
layers conformed to the prediction.
[0056] Noting that PEO is substantially less permeable to oxygen
than EAA, equation (1) predicts that P// will be quite sensitive to
P.sub.PEO even if .phi.PEO is relatively small. To ascertain
whether a dramatic change in the PEO permeability was responsible
for the layer thickness effect, the permeability of numerous films
that varied in both the composition ratio and the layer thickness
was tested. Although the measured values of P scattered, depending
on the composition ratio, when an effective P.sub.PEO,eff was
extracted by assuming
P PEO , eff = .theta. PEO ( 1 P + 1 - .theta. PEO P EAA ) - 1
##EQU00003##
the data collapsed to a single curve when P.sub.PEO.eff was plotted
as a function of the PEO layer thickness, FIG. 9. Only the results
for the thicker PEO microlayers conformed to equation (2) with
P.sub.PEO.eff.apprxeq.PEO, as indicated by the dashed line.
Deviation below the line was seen with 1 .mu.m-thick PEO layers.
The lowest value of P.sub.PEO.eff was 0.0052 barrer, which was
almost 2 orders of magnitude less than P.sub.PEO.
[0057] Crystals are generally considered to be impermeable to small
gas molecules, and gas transport is seen as occurring through the
amorphous regions of the polymer. It seemed likely that
crystallization in a confined space resulted in an unusual
crystalline morphology that endowed the PEO nanolayers with
exquisitely low permeability. However, differential scanning
calorimetry revealed that even in the thinnest layers, both PEO and
EAA possessed the same melting enthalpy and the same melting
temperature as the non-layered film controls, which were 153 J/g
and 66.degree. C. for PEO, and 98 J/g and 98.degree. C. for EAA,
respectively. Thus, any unusual crystalline morphology that
provided the very low permeability of PEO nanolayers was not
accompanied by changes in the 70% level of crystallinity or in the
lamellar thickness.
[0058] The layers were viewed directly by microtoming the film
through the thickness and examining the exposed surface in the
atomic force microscope (AFM). A region from the cross-section of a
film with 3.6 .mu.m-thick PEO layers and EAA/PEO 50/50 composition
confirmed that the layers were well-defined and continuous, FIG.
10A. The PEO had substantially higher crystallinity than the EAA,
and hence the PEO layers appeared bright in the AFM images.
Although there was some nonuniformity, the average layer thickness
was close to the nominal layer thickness calculated from the film
thickness, the composition ratio and the total number of layers. A
higher magnification image showed the sharp boundaries between EAA
and PEO layers, and revealed the spherulitic morphology of the PEO
layer, FIG. 10B. The spherulites appeared to nucleate predominantly
within the PEO layer and only occasionally from the interface.
There was no apparent orientation of the morphological features
relative to the layered structure. Rather, the morphology closely
resembled the isotropic spherulitic morphology of PEO crystallized
from the unconfined melt. It was expected that the properties of
the PEO layers would also be the same, and indeed, the oxygen
permeability of films with thick PEO layers closely conformed to
equation (2) with PP.sub.EO.eff.apprxeq.PPEO.
[0059] Another pair of images in FIGS. 10C-D compares a film with
110 nm-thick PEO layers and, in this case, an EAA/PEO 70/30
composition. Because the layer thickness was orders of magnitude
lower than in FIGS. 10A-B, the scale of the AFM images is
different. Again, the images confirmed the continuity of the thin
PEO layers and the close correspondence between the average layer
thickness and the nominal thickness. At higher magnification, the
effect of confinement on crystallization of the PEO layer was
apparent. The PEO crystallized as stacks of three to five long thin
lamellae oriented in the plane of the layer.
[0060] When the PEO layer thickness was reduced to 20 nm, most of
the PEO layers crystallized as single, extremely long lamellae,
FIG. 10E. In contrast to the lamellar stacks in 110 nm layers, the
length of the single lamellae in 20 nm layers frequently exceeded
the dimension of the AFM images. Due to the variation in layer
thickness, an occasional PEO layer was thick enough to crystallize
with two parallel lamellae. Coincidence between the layer
thickness, which was determined by the extrusion conditions, and
the thickness of PEO lamellae, about 20 nm, facilitated
crystallization of the layers as single lamellae. The individual
PEO layers could be thought of as very large single crystals.
[0061] If the layer thickness was reduced to 8 nm, the layers broke
up. This was confirmed with AFM images. It was possible that
breakup occurred during crystallization, driven by crystallization
of lamellae with thickness greater than the layer thickness, rather
than by interfacial driven breakup of the melt. Layer breakup was
the cause of the increased permeability of the film with 8 nm PEO
layers.
[0062] The unique crystalline morphology was responsible for the
very low gas permeability of 20 nm PEO layers. For a continuous
single crystal, the fold surfaces constitute the permeable
amorphous regions. A diffusion pathway through the impermeable
crystalline core depends on the frequency of defects such as
lamellar edges. Structurally, the nanolayered assembly resembles a
dispersion of impermeable platelets of given aspect ratio, which
has been modeled by Cussler et al. (E. L. Cussler, S, E. Hughes, W.
J., m Ward, R. Aris,. J. Membr. Sci, 38,161-174 (1988)). If the
platelets are oriented perpendicular to the flux, the permeability
of the composite is expressed as
P = P EAA [ 1 + .alpha. 2 .theta. 2 4 ( 1 - .theta. ) ]
##EQU00004##
[0063] where .phi. is the volume fraction of impermeable platelets
and .alpha. is the aspect ratio of the platelets defined as length
divided by width. In this case, .phi. was taken as the volume
fraction of the PEO layers multiplied by the volume fraction of the
PEO crystalline phase. For PEO nanolayers, the aspect ratio from
equation (3) was as high as 120, which meant a lateral dimension of
more than 2 .mu.m for lamellae 20 nm thick. This exceeded the
dimension of the AfM image, which explained why the lamellae in
FIG. 10E often appeared to be continuous.
Single Crystal Texture of Confined PEO Layers
[0064] Confirmation of the oriented lamellar morphology and details
of the global orientation were obtained with small angle X-ray
scattering (SAXS) and wide angle X-ray scattering (WAXS). The SAXS
examines the periodic arrangement of lamellar crystals within the
constituent layers. By aligning the incident X-ray beam parallel to
the normal direction (ND), the extrusion direction (ED) and the
transverse direction (TD), the particular orientation of lamella
was determined from the corresponding patterns (see supporting
material). The scattering patterns indicated that the long spacing
of the PEO and EAA lamellae in layered films remained nearly the
same as in the control films and they were 22.+-.0.6 nm and
10.8.+-.0.5 nm, respectively. Isotropic scattering patterns in all
three directions from 3.6 .mu.m PEO layers indicated that the PEO
layer was too thick for PEO lamellae to feel any significant
confinement effect. However, as the PEO layer thickness decreased
to 110 nm, highly oriented meridianal two-point scattering features
of the stacked PEO lamellae appeared in the ED and TD patterns,
which indicated that PEO lamellae were oriented and stacked
primarily parallel to the layer surface due to the confinement
effect. Scattering from the 20 nm PEO layers further confirmed the
single population of in-plane lamellae that had grown in the
direction parallel to the PEO layers. The extremely weak
first-order peak from the PEO layers indicated that they existed
predominantly as single lamellae, rather than as stacked lamellae,
as observed in FIG. 10E. It was thought that the observed weak
lamellar correlation peak from 20 nm PEO layers was associated with
the thickness distribution of the layers, which occasionally
enabled formation of two single crystals in a single PEO layer.
[0065] The orientation of PEO chains in crystal was examined by
using 2D WAXS and pole figure technique. Consistent results were
obtained by these two techniques (see supporting material for 2D
WAXS). In FIG. 11, the pole figures of normals to (120) and (032)
planes of PEO are presented for the PEO control film and three
EAA/PEO layered films. From FIG. 4A it is seen that there is no
preferred orientation of PEO crystals except for very faint
orientation due to extrusion direction (vertical). In FIG. 11B, the
film with 3.6 .mu.m-thick PEO layers showed a very weak orientation
of PEO crystals, which in fact could be identified as an artifact
due to slight defocusing of the X-ray beam when the specimen was
tilted during data collection. In contrast, films with 110 nm-thick
PEO layers showed a very strong orientation of (120) and also (032)
planes as seen in FIG. 11C. Nearly all the (120) planes that
contain macromolecular chains are perpendicular to the film plane.
This means that the fold surfaces of the lamellar PEO crystals are
parallel to the layer interfaces. Upon decreasing the PEO layer
thickness to 20 nm, the preferred orientation of PEO lamellae
parallel to the layers seems even stronger as can be judged from
the narrower ring at the pole figure circumference, FIG. 11D. The
(120) planes were distributed evenly in the plane of film, always
being perpendicular to the film surface. The (032) planes of PEO
crystals are tilted by 67.degree. from the chain axis (22). The
pole figures for (032) normals in FIGS. 11C-D resemble rings
exactly off-set by 67.degree. as predicted by the crystallographic
unit cell for orientation of PEO lamellae parallel to the layer
interface. Again the ring for (032) normals in FIG. 11D is much
narrower than that in FIG. 11C, which indicates better orientation
of PEO lamellae parallel to the layer interfaces.
[0066] The crystal orientation of PEO in confined nanolayers
essentially reproduced the crystal structure reported in
self-assembled PEO blocks in PS-b-PEO diblock copolymers. Comparing
the sharpness of the WAXS pattern, higher orientation was achieved
by physically confining a high molecular weight PEO between
force-assembled layers than by confining a low molecular weight PEO
block between self-assembled lamellae with covalent links. When the
thickness confinement occurred on the size scale of the usual
lamellar thickness, the PEO layers crystallized as single lamellae
with extremely large aspect ratio. It was suggested that the
lamellae could be thought of as large, impermeable single crystals.
This may be the first time that large polymer single crystals were
obtained by melt processing.
[0067] The coextrusion process, which operates with polymers that
are readily available, now makes it possible to fabricate
nanolayered polymeric structures in quantities sufficient to probe
the structure-property relationships of the unique morphologies
resulting from nanoscale confinement. Polymer nanolayers can be
incorporated into conventional polymeric films to utilize their
unique properties in the design and execution of packaging
strategies that address growing environmental and energy
concerns.
[0068] SAXS analysis in FIG. 12 shows the 2-dimensional SAXS
patterns of EAA/PEO films with 3.6 .mu.m, 110 nm and 20 nm PEO
layers where the incident X-ray beam was parallel to the normal
direction (ND) and to the extrusion direction (ED). Because the
SAXS patterns measured in the transverse direction (TD) were
indistinguishable from those in the ED, only ED and ND patterns arc
presented in the following discussion. The intense meridional
streak in the ED patterns was mainly associated with grazing
incidence scattering. This scattering was found to veil weak
scattering from the lamellae. For clarification, the equatorial and
meridianal scattering profiles were extracted from the 2D patterns.
The peak assignments were based on the peak positions of the PEO
and EAA control films. Comparison of the various scattering
profiles indicated that the first-order peak positions of the PEO
and EAA lamellae in coextruded EAA/PEO films remained nearly the
same as in the control films. The long periods, Lp=2.pi./q, for PEO
and EAA lamellae obtained from the SAXS measurements were 22.+-.0.6
nm and 10.8.+-.0.5 nm, respectively. The long period of PEO was
consistent with literature reports for this molecular weight.
[0069] The ND and ED profiles from 3.6 .mu.m PEO layers showed
almost the same peak sharpness and height as the PEO control
implying that the PEO layers were too thick for PEO lamellae to
feel any significant confinement effect. The slight increase in the
meridianal intensity over the equatorial intensity in the ND
patterns was attributed to the melt flow during coextrusion.
[0070] As the PEO layer thickness decreased to 110 nm and 20 nm,
however, highly oriented scattering features of the PEO lamellae
appeared. These were due to the spatial confinement, not to a
mechanical flow effect. The scattering peak of PEO lamellae in the
EO meridianal pattern was much stronger and sharper than in the ED
equatorial pattern, where it was barely discernable. Also, no
first-order peak maximum for the PEO lamellae was discerned in the
ND patterns. These scattering features implied that large-scale,
oriented structures with the main scattering vector normal to the
layer formed in the PEO layers. This was evidence that PEO lamellae
were oriented primarily parallel to the layer surface.
Crystallization as in-plane lamellae was due to the narrow
confinement in the EAA layer interstices. The well-oriented
in-plane lamellae were not detected when the X-ray beam was
parallel to the ND since this direction was along the projection
direction of the lamellar stacks. The in-plane lamellae were
totally different from the individual lamellae of a shish-kebab or
a spherulite, which have only one growth face. Rather, they were
more like single crystals.
[0071] Scattering from the 20 nm PEO layers was only detected on
the meridian with the X-ray beam parallel to the ED. This implied a
single population of in-plane lamellae that had grown in the
direction parallel to the PEO layers. It was also noted that the
first order peak maxima in the SAXS pattern from 20 nm layers was
much weaker than that from 110 nm layers. If the lamellae were
uncorrelated within the PEO layers, they should not show a
first-order peak maximum in the SAXS pattern, and rather, should
exhibit only single lamellar scattering features. Thus, it was
thought that the observed weak lamellar correlation peak from 20 nm
PEO layers was associated with the thickness distribution of the
layers, which occasionally enabled formation of two single crystals
in a single PEO layer.
[0072] In contrast to the high degree of lamellar orientation in
the PEO layers, the broad EAA first order SAXS reflection appeared
in the ED and TD patterns with only a slight meridianal
concentration, which was especially evident in the ND profiles. The
slight residual orientation of the EAA lamellae was attributed to
the melt flow during eoextrusion.
[0073] WAXS patterns from EAA and PEO films confirmed that EAA had
the orthorhombic crystal form of polyethylene, and PEO took the
usual monoclinic crystal form. The WAXS patterns of EAA/PEO layered
films are shown in FIG. 13. Because the WAXS patterns measured in
the TD were indistinguishable from those in the ED, only ED and ND
patterns are presented here, The EAA (110) reflection (scattering
angle 2.theta.=21.5.degree., and the PEO (120) reflection
(2.theta.=19.2.degree. and (032) reflection (2.theta.=23.3.degree.
appear in the WAXS patterns (The PEO reflection labeled (032)
actually contains reflections from a group of planes which have
similar d-spacing. A detailed assignment can be found in L. Zhu,
et. al. J. Am. Chem. Soc. 122, 5957-5967 (2000)). The EAA (200)
reflection (2.theta.=23.6.degree., which was seen in the EAA
pattern, was superimposed on the stronger PEO (032) reflection. The
ND and TD patterns from the coextruded films with 3.6 .mu.m-thick
PEO layers exhibited almost isotropic rings for reflections from
both the PEO and the EAA layers, FIG. 13A.
[0074] The ED pattern from the film with 110 nm PEO layers (FIG.
13B) revealed considerable orientation of the PEO. The PEO (120)
reflections appeared as equatorial arcs and the PEO (032)
reflections as arcs at approximately +65.degree. and -65.degree.
with respect to the vertical direction. In addition, the (224) and
(024) reflections appeared at +45.degree. and -45.degree. with
respect to the vertical direction. Decreasing the PEO layer
thickness to 20 nm sharpened the arcs in the ED pattern to spots,
FIG. 13C. The ED pattern resembled the PEO fiber pattern (Y.
Takahashi, H. Tadokoro, Macromolecules 6,672-675 (1973)) and
indicated that the c-axis of the PEO crystals was oriented along
the ND, i.e. vertical to the layer plane.
[0075] The ND patterns from the 110 nm and 20 nm PEO layers showed
strong scattering at all angles, suggesting that the lamellae were
essentially randomly oriented in the layer plane. In contrast to
the high degree of orientation in the PEO layers, the EAA (110)
reflection appeared as an almost isotropic ring in the ED patterns
with only a slight equatorial concentration in the ND pattern due
to slight orientation of the EAA chains in the extrusion direction.
No other specific orientation of the EAA crystallographic planes
was observed, although the BAA layers in these two samples were
less than 400 nm thick. It was highly unlikely that the slight
residual orientation of EAA from the extrusion process affected the
oxygen permeability.
[0076] While a preferred embodiment of the invention has been
illustrated and described, it shall be understood that the
invention is not limited to this embodiment. Numerous
modifications, changes and variations will be obvious for those
skilled in the art, without departing from the scope of the
invention as described by the appended claims. All patents,
publications, and references cited herein are incorporated by
reference in their entirety.
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